1. Field of the Invention
The present invention relates to a sensor comprising a magnetoresistive element and used as a magnetic head, a potentiosensor, an angular sensor, and the like, a manufacturing method thereof and a magnetic head comprising the sensor.
2. Description of the Related Art
As magnetoresistive reading heads (MR heads), AMR (Anisotropic Magnetoresistive) heads using the anisotropic magnetoresistive effect, and GMR (Giant Magnetoresistive) heads using spin dependent scattering are conventionally known. An example of the GMR heads is the spin-valve head disclosed in U.S. Pat. No. 5,159,513 in which high magnetoresistance is exhibited in a low magnetic field.
FIGS. 10 and 11 are drawings respectively showing the schematic constructions of AMR head element structures.
The head element shown in FIG. 10 comprises an insulation layer 2 and a ferromagnetic layer (AMR material layer) 3 which are laminated on a soft magnetic layer 1, antiferromagnetic layers 4 which are laminated at both ends of the ferromagnetic layer 3 with a space therebetween corresponding to a track width, and electrically conductive layers 5 respectively laminated on the antiferromagnetic layers 4. The head element shown in FIG. 11 comprises a soft magnetic layer 1, an insulation layer 2 and a ferromagnetic layer 3 which form a laminate, hard magnetic layers 6 provided on both sides of the laminate to hold it therebetween, and electrically conductive layers 5 respectively provided on the hard magnetic layers 6.
For optimum operation of such AMR heads, two magnetic bias fields are required for the ferromagnetic layer 3 exhibiting the AMR effect.
A first magnetic bias field functions to make the resistance of the ferromagnetic layer 3 change in linear response to a magnetic flux from a magnetic recording medium. The first magnetic bias field is perpendicular (in the Z direction shown in FIG. 1) to the surface of the magnetic recording medium and parallel to the film surface of the ferromagnetic layer 3. The first magnetic bias field is generally referred to as a xe2x80x9clateral biasxe2x80x9d and can be obtained by flowing a sensing current through the AMR head element from the electrically conductive layers 5.
A second magnetic bias field is generally referred to as a xe2x80x9clongitudinal biasxe2x80x9d and applied in parallel (in the X direction shown in FIG. 1) with the film surface of the ferromagnetic layer 3. The longitudinal magnetic bias field is applied for suppressing the Barkhausen noise produced due to the formation of many magnetic domains in the ferromagnetic layer 3, i.e., causing the resistance to smoothly change with the magnetic flux from the magnetic recording medium with less noise.
However, in order to suppress the Barkhausen noise, it is necessary to put the ferromagnetic layer into a single magnetic domain state. As a method of applying the longitudinal bias for this purpose, the following two methods are generally known. A first method uses the head element structure shown in FIG. 11 in which the hard magnetic layers 6 are disposed on both sides of the ferromagnetic layer 3 to employ a leakage magnetic flux from the hard magnetic layers 6. A second method uses the head element structure shown in FIG. 10 in which the exchange anisotropic magnetic field produced in the contact boundary surfaces between the antiferromagnetic layers 4 and the ferromagnetic layer 3 is employed.
As an element structure which employs exchange anisotropic coupling due to the antiferromagnetic layers, the exchange bias structure shown in FIG. 12, and the spin-valve structure shown in FIG. 13 are known.
The structure shown in FIG. 12 is classified as the structure shown in FIG. 10, and comprises a ferromagnetic layer 22, a non-magnetic layer 23 and a ferromagnetic layer 24 exhibiting the magnetoresistive effect which are laminated on a lower insulation layer 21, antiferromagnetic layers 25 and lead layers 26 which are provided on both sides of the ferromagnetic layer 24 with a space corresponding to the track width TW, and an upper insulation layer 27 provided on these layers.
In the structure shown in FIG. 12, a longitudinal bias is applied to the ferromagnetic layer 24 due to the exchange anisotropic coupling in the boundaries between the ferromagnetic layer 24 and the antiferromagnetic layers 25 to put regions B (the regions where the ferromagnetic layer 24 contacts the antiferromagnetic layers 25 ) shown in FIG. 12 into a single magnetic domain state in the X direction. This brings region A of the ferromagnetic layer 24 within the track width into a single magnetic domain state in the X direction. A sensing current is supplied to the ferromagnetic layer 24 from the lead layers 26 through the antiferromagnetic layers 25. When the sensing current is supplied to the ferromagnetic layer 24, a lateral magnetic bias field in the Z direction is applied to the ferromagnetic layer 24 due to the magnetostatic coupling energy from the ferromagnetic layer 22. In this way, when the leakage magnetic filed is applied to the ferromagnetic layer 24 magnetized by the longitudinal magnetic bias field and the lateral magnetic bias field from the magnetic recording medium, the electric resistance to the sensing current linearly responds to the magnitude of the leakage magnetic field and changes in proportion thereto. Therefore, the leakage magnetic field can be sensed by a change in the electric resistance.
The structure shown in FIG. 13 comprises a free ferromagnetic layer 28, a non-magnetic electrically conductive layer 29 and a ferromagnetic layer 24 which are laminated to form a magnetoresistive element 19, and an antiferromagnetic layer 25 and an upper insulation layer 27 which are laminated in turn on the ferromagnetic layer 24.
In the structure shown in FIG. 13, the sensing current is supplied to the magnetoresistive element 19. The magnetization of the ferromagnetic layer 24 is fixed in the Z direction due to exchange anisotropic coupling with the antiferromagnetic layer 25. Therefore, when a leakage magnetic field is applied from a magnetic recording medium which is moved in the Y direction, the electric resistance of the magnetoresistive element 19 changes with a change in the magnetization direction of the free ferromagnetic layer 28, and the leakage magnetic field can thus be sensed by this change in the electric resistance.
Other known structures for optimum operation of the above structures by employing the spin valve structure include the structure shown in FIG. 14 which comprises a free ferromagnetic layer 7, a non-magnetic buffer layer 8, a pinned ferromagnetic layer 9 and an antiferromagnetic layer 10, which are laminated in turn to form a laminate, hard magnetic layers 11 which are provided on both sides of the laminate, and electrically conductive layers 12 respectively provided on the hard magnetic layers 11, and the structure shown in FIG. 15 which comprises a free ferromagnetic layer 7, a non-magnetic buffer layer 8, a pinned ferromagnetic layer 9 and an antiferromagnetic layer 10, which are laminated in turn to form a laminate, an electrically conductive layer 12 and an antiferromagnetic layer 13 which are provided on the upper and lower sides of the laminate to hold it therebetween at either side thereof, and a buffer layer 14 provided adjacent to the whole laminate.
In the structure shown in FIG. 14, it is necessary that the magnetization direction of the free ferromagnetic layer 7 is directed in the track direction (the X direction shown in FIG. 14) in the state where a bias in the track direction is applied to the free ferromagnetic layer 7 to put it into a single magnetic domain state by the hard magnetic layers 11, and that the magnetization direction of the pinned ferromagnetic layer 9 is directed in the Z direction shown in FIG. 14, i.e., the direction perpendicular to the magnetization direction of the free ferromagnetic layer 7, in the state where a bias is applied in the Z direction to put the pinned ferromagnetic layer 9 into a single magnetic domain state. In other words, the magnetization direction of the pinned ferromagnetic layer 9 must not be changed by a magnetic flux (in the Z direction shown in FIG. 14) from the recording magnetic medium, and the magnetization direction of the free ferromagnetic layer 7 is changed within the range of 90xc2x10xc2x0 with the magnetization direction of the pinned ferromagnetic layer 9 to obtain linear response of magnetoresistance.
In order to fix the magnetization direction of the pinned ferromagnetic layer 9 in the Z direction shown in FIGS. 14 and 15, a relatively large bias magnetic field is required, and this bias magnetic field is preferably as large as possible. In order to overcome an antiferromagnetic field in the Z direction shown in FIGS. 14 and 15, and avoid fluctuation of the magnetization direction due to the magnetic flux from the recording magnetic medium, a bias magnetic field of at least 100 Oe is required.
In the structures shown in FIGS. 14 and 15, this bias magnetic field is obtained by using the exchange anisotropic coupling produced by providing the pinned ferromagnetic layer 9 and the antiferromagnetic layer 10 in contact with each other.
The bias applied to the free ferromagnetic layer 7 is adapted for securing linear response and suppressing the Barkhausen noise produced due to the formation of many magnetic domains. Like the longitudinal bias in an AMR head, the structure shown in FIG. 14 uses as the bias the leakage magnetic flux from the hard magnetic layers 11 which are provided on both sides of the free ferromagnetic layer 7. The structure shown in FIG. 15 uses as the bias the exchange anisotropic magnetic field produced in the contact boundary surfaces between the free ferromagnetic layer 7 and the antiferromagnetic layers 13 provided on both sides of the free ferromagnetic layer 7.
As described above, the exchange anisotropic magnetic field produced in the contact boundary with the antiferromagnetic layers is used as the longitudinal bias in the AMR head, the bias for the pinned ferromagnetic layer in a spin valve head, and the bias for the free ferromagnetic layer. As result, a magnetoresistive head exhibiting good linear response and the effect of suppressing Barkhausen noise is realized.
The exchange anisotropic magnetic field is the phenomenon caused by exchange interaction between the magnetizing moments of the ferromagnetic layer and the antiferromagnetic layer in the contact boundary layer therebetween. As the antiferromagnetic layer producing the exchange anisotropic magnetic field with the ferromagnetic layer, e.g., an NiFe layer, an FeMn layer is well known. However, the FeMn layer has a problem in that since it has low corrosion resistance, corrosion proceeds in the process of manufacturing a magnetic head and in operation of the magnetic head, thereby deteriorating the exchange anisotropic magnetic field, and damaging the recording magnetic medium in some cases. It is known that the temperature in the vicinity of the FeMn layer during operation of the magnetic head readily increases to about 120xc2x0 C. by heat of the stationary sensing current. However, the exchange anisotropic magnetic field produced by the FeMn layer is extremely sensitive to a temperature change, and substantially linearly decreases with a temperature increase to about 150xc2x0 C. at which it disappears (blocking temperature: Tb). There is also a problem in that a stable exchange anisotropic magnetic field cannot be obtained.
On the other hand, as an invention of improvements in the corrosion resistance and blocking temperature of an FeMn film, for example, the NiMn alloy or NiMnCr alloy having a face-centered tetragonal structure disclosed in U.S. Pat. Nos. 5,315,468 and 5,436,778 is known. However, the corrosion resistance of an NiMn layer is higher than that of the FeMn layer, but is insufficient for practical use. An NiMnCr layer contains Cr which is added for improving the corrosion resistance of the NiMn layer, but has a problem in that although the corrosion resistance is improved by adding Cr, the magnitude of the exchange anisotropic magnetic field and the blocking temperature are decreased.
Further, in order to obtain the exchange anisotropic magnetic field in the NiMn alloy or NiMnCr alloy, it is necessary to form a CuAg-I type ordered structure crystal having the face-centered tetragonal (fct) structure in a portion of the antiferromagnetic layer, and it is, of course, necessary to control ordered-disordered transformation and the volume ratio of ordered phase and disordered phase. Therefore, there is a problem in that in order to obtain stable properties, control and management of the process for manufacturing a magnetic head must be significantly complicated. There are also problems in that in order to obtain the necessary exchange anisotropic magnetic field, heat treatment in a magnetic field must be repeated several times, and in that the temperature must be decreased at a low rate, for example, a time required for decreasing the temperature from 255xc2x0 C. to 45xc2x0 C. is 17 hours (refer to Appl. Phys. Lett., 65(9), Aug. 29, 1994). Thus the treatment time in the manufacturing process is increased, and the efficiency of manufacture deteriorates.
As an invention of improvement in the blocking temperature of the FeMn layer, a method is disclosed in U.S. Pat. No. 4,809,109 in which a NiFe/FeMn laminated film is heat-treated at a temperature of 260 to 350xc2x0 C. for 20 to 50 hours to form a Nixe2x80x94Fexe2x80x94Mn three-element alloy layer in the NiFe/FeMn boundary surface due to diffusion by heat treatment. However, it can be understood that this method has no effect on improvement in corrosion resistance which is the greatest problem, and this method has a problem in that the required heat treatment time is as long as 20 to 50 hours, and thus deteriorates the efficiency of manufacture.
On the other hand, Mn system alloys such as NiMn, PtMn, AuMn, RhMn3 and the like are shown as antiferromagnetic materials in an existing publication, e.g., xe2x80x9cMagnetic Material Handbookxe2x80x9d issued by Asakura Shoten. However, there is no comment about an exchange anisotropic magnetic field in the contact boundary surface with the ferromagnetic layer, and characteristics of an antiferromagnetic layer itself and exchange anisotropic magnetic field in a super thin film having a thickness of several hundreds A are not entirely clear.
In the element structure shown in FIG. 14, the free ferromagnetic layer 7 to which the bias is applied by the right and left hard magnetic layers 11 tends to become an insensitive region where the magnetization direction in the track end portions (the region denoted by reference numeral 16 in FIG. 14) near the hard magnetic layers 11 is hardly changed. Therefore, when the track width is decreased with improvement in the recording density of the recording magnetic medium, this structure possibly causes a problem.
The element structure using an exchange coupling bias shown in FIG. 15 can thus become promising, but the structure has the problem below when a longitudinal bias is applied to the spin valve element structure by the exchange coupling bias method.
In the spin valve element structure shown in FIG. 15, rotation of magnetization of the pinned ferromagnetic layer 9 is fixed by the antiferromagnetic layer 10, while the magnetization direction of the track end portions of the free ferromagnetic layer 7 is fixed for the longitudinal bias by the antiferromagnetic layers 13. A difference between the magnetization directions in which the antiferromagnetic layer 10 and the antiferromagnetic layers 13 are respectively fixed is 90xc2x0.
The magnetization direction of each of the magnetic layers is generally controlled by deposition in a magnetic field or annealing in a magnetic field after deposition. However, it is very difficult to control the magnetization direction of the antiferromagnetic layer 10 which is deposited after the antiferromagnetic layers 13 are deposited, without disturbing the magnetization direction of the antiferromagnetic layers 13.
Also a technique is disclosed in U.S. Pat. No. 5,528,440 in which the above problems are solved by using magnetic films having different Neel temperatures and employing different heat treatment temperatures for the respective magnetic films. However, this technique also has the need for using an FeMn alloy having a low Neel temperature, and thus has a problem in that the drawback of low corrosion resistance and the drawback of sensitivity to a temperature change due to the blocking temperature cannot be solved.
The present invention has been achieved in consideration of the above situation, and an object of the invention is to provide a magnetoresistive sensor with excellent corrosion resistance and linear response in which a necessary sufficient exchange anisotropic magnetic field can be applied in a thin film, and Barkhausen noise is suppressed.
Another object of the present invention is to provide a magnetoresistive sensor in which by providing an antiferromagnetic layer having a high blocking temperature, the linear response and resistance to temperature changes are improved and Barkhausen noise is suppressed.
A further object of the present invention is to provide a method of manufacturing a magnetoresistive sensor having the above excellent properties, which has no need for heat treatment in a magnetic field for a long time using special heat treatment equipment, which can manufacture the sensor by usual heat treatment, and which can reduce the heat treatment time, as compared with a conventional method.
In order to achieve the objects of the present invention, there is provided a magnetoresistive sensor comprising at least two ferromagnetic layers provided with a non-magnetic layer therebetween; a coercive force increasing layer comprising a first antiferromagnetic layer provided adjacent to one of the ferromagnetic layers, for increasing the coercive force of that ferromagnetic layer to pin magnetization reversal thereof, the other ferromagnetic layer having freed magnetization reversal; and a second antiferromagnetic layer comprising an antiferromagnetic material provided adjacent to the other ferromagnetic layer having freed magnetization reversal, for applying a longitudinal bias to the other ferromagnetic layer to induce unidirectional magnetic anisotropy to stabilize a magnetic domain.
In the present invention, the magnetization direction of the ferromagnetic layer having pinned magnetization reversal is preferably substantially perpendicular to the magnetization direction of the ferromagnetic layer having free magnetization without an external magnetic field.
In the present invention, the coercive force increasing layer comprises xcex1-Fe2O3, and the coercive force of the ferromagnetic layer having magnetization reversal pinned by the coercive force increasing layer is preferably higher than the unidirectional exchange bias magnetic field simultaneously induced in the ferromagnetic layer by xcex1-Fe2O3.